The present invention relates to optical dispersion compensation and optical pulse manipulation, and more specifically, to devices and systems having an optical grating capable of causing wavelength-dependent delays.
Dispersion in optical waveguides such as optical fibers causes optical waves of different wavelengths to travel at different speeds. One parameter for characterizing the dispersion is group velocity which is related to the derivative of the propagation constant of an optical wave with respect to frequency. The first-order group velocity dispersion is typically expressed as a change in light propagation time over a unit length of fiber with respect to a change in light wavelength. For many fibers used in telecommunication, the first-order group velocity dispersion is on the order of 10 ps/nm/km at 1550 nm.
In many applications, an optical signal is composed of spectral components of different wavelengths. For example, a single-frequency optical carrier may be modulated in order to impose information on the carrier. Such modulation generates modulation sidebands at different frequencies from the carrier frequency. For another example, optical pulses, which are widely used in optical data processing and communication applications, contain spectral components in a certain spectral range. The dispersion effect may cause adverse effects on the signal due to the different delays on the different spectral components.
Dispersion in particular presents obstacles to increasing system data rates and transmission distances without signal repeaters in either single-channel or wavelength-division-multiplexed (xe2x80x9cWDMxe2x80x9d) fiber communication systems. Data transmission rates up to 10 Gbit/s or higher may be needed in order to meet the increasing demand in the marketplace. Dispersion can be accumulated over distance to induce pulse broadening or spread. Two adjacent pulses in a pulse train thus may overlap with each other at a high data rate. Such pulse overlapping can cause errors in data transmission.
One way to reduce the dispersion effect in fibers is to implement a fiber grating with linearly chirped grating periods. The resonant wavelength of the fiber grating changes with the position due to the changing grating period. Therefore, different spectral components in an optical signal are reflected back at different locations and thus have different delays. Such wavelength-dependent delays can be used to reduce the accumulated dispersion in a fiber link.
The present disclosure includes techniques and devices based on a wave-guiding element which has a spatial grating pattern that is an oscillatory variation along its optic axis. The wave-guiding element is configured to receive an input optical signal and to produce an output optical signal by reflection within a Bragg reflection band produced by the spatial grating pattern so as to produce time delays of different reflected spectral components as a nonlinear function of spatial positions along said optic axis at which the different reflected spectral components are respectively reflected. A control unit may be engaged to the wave-guiding element and is operable to change a property of the spatial grating pattern along the optic axis to tune at least relative time delays of the different reflected spectral components nonlinearly with respect to wavelength. The dispersion of such a wave-guiding element can be dynamically adjusted to produce a desired dispersion with desired relative delays among different spectral components in a controllable manner.
One embodiment of the above wave-guiding element is the nonlinearly-chirped grating which may include a grating that has an effective index neff(x) and the grating period xcex9(x) are configured to produce a grating parameter neff(x)xcex9(x) as a nonlinear function of the position along the fiber optic axis. Such a grating reflects optical waves satisfying a Bragg condition of xcex(x)=2neff(x)xcex9(x). A single Bragg reflection band is generated where the bandwidth is determined by the chirping range of the grating parameter neff(x)xcex9(x).
A grating tuning mechanism may be implemented by using a grating control unit to control either the effective index neff(x) or the grating period xcex9(x). This allows for adjustment of the grating parameter neff(x)xcex9(x) and thus to the relative delays for signals at different wavelengths within the bandwidth of the reflection. A transducer, e.g., a piezoelectric element, may be used as the control unit to compress or stretch the overall length of the grating in order to produce a tunable dispersion profile. A magnetostrictive element may also be used to change the grating length according to an external control magnetic field. If the grating material is responsive to a spatially-varying external control field such as an electric field, an electromagnetic radiation field, or a temperature field along the grating direction, a control unit capable of producing such conditions can be used to change effective index of refraction and to produce a tunable dispersion profile.
In addition, the frequency response of a nonlinearly chirped grating may be tuned by using an acoustic wave propagating along the grating direction. The acoustic wave induces additional modulation sidebands in the frequency response of the grating. Such modulation sidebands are displaced from the baseband by a frequency spacing that is dependent on the frequency of the acoustic wave. Therefore, an adjustable dispersion can be achieved by tuning the frequency of the acoustic wave.
The present disclosure also provides a sampled nonlinearly-chirped grating for changing relative time delays of signals at different wavelengths. This sampled nonlinearly-chirped grating includes a wave-guiding element having a refractive index that varies along its optic axis according to a multiplication of a first spatial modulation and a second spatial modulation. The first spatial modulation is an oscillatory variation with a nonlinearly-chirped period along the optic axis. The second spatial modulation is a periodic modulation with a period different than the nonlinearly-changing period.
The first and second modulations effectuate first and second gratings that spatially overlap each other in the wave-guiding element along its optic axis. The first grating may be a nonlinearly-chirped grating. The second grating may have a grating period greater than the first grating. The first grating and second gratings couple with each other and operate in combination to produce a plurality of reflection bands at different wavelengths and with a bandwidth determined by the first grating.
A nonlinearly-chirped grating can be further configured to change relative time delays of two different polarization states in an optical signal. One embodiment of such a grating comprises a wave-guiding element formed of a birefringent material that exhibits different refractive indices for the two polarization states. A nonlinearly-chirped grating is formed in the wave-guiding element along its optic axis and has a varying grating period that changes as a monotonic nonlinear function of a position. The grating operates to reflect two polarization states of an input optical signal at different locations along the optic axis to cause a delay between said two polarization states.
One aspect of the nonlinearly-chirped gratings is dispersion compensation. A nonlinear chirped grating can be disposed at a fiber link to reduce the effects of the dispersion. The dispersion produced by such a grating is actively tunable to compensate for varying dispersion in a fiber link which includes a dispersion analyzer and a feedback control. This tunability can be advantageously used in a dynamic fiber network in which communication traffic patterns may change over time. For example, a given channel may be originated at different locations in the network from time to time so that the accumulated dispersion of that given channel in a specific fiber link is a variable. Therefore, the dispersion compensation required for that fiber link needs to change accordingly. Also, the operating conditions for point-to-point transmission may also change, resulting in variations in the accumulated dispersion for signals in a fixed fiber link.
Another aspects of the nonlinearly-chirp gratings include dispersion slope compensation, polarization mode dispersion, chirp reduction in directly modulated diode lasers, and optical pulse manipulation.
These and other embodiments, aspects and advantages of the invention will become more apparent in light of the following detailed description, including the accompanying drawings and appended claims.